Latch assemblies for doors commonly include a turn knob or handle which is generally located at the inside of the door and which is rotated to withdraw the latch bolt into its casing. In order to improve the security of such assemblies, manufacturers have included a key operated lock which is operative to releasably hold the turn knob or handle against rotation. Such locks are typically arranged so as to be operated from the inside of the door and are not operable from the outside of the door. In particular, key operation of the latch from the outside of the door will not release the lock. That can lead to serious problems in circumstances where the door needs to be opened urgently from the inside, particularly if the lock key has been misplaced or is not conveniently accessible.
It is an object of the present invention to provide a key controlled latch which can be released from a locked condition by use of a key at the outside of the door or other member with which the latch is used.
A typical latch assembly to which the present invention is applicable includes a casing, a latch bolt movable relative to the casing to adopt either an extended latching position or a retracted release position, a first actuator at an inner side of the assembly which is operable to move the latch bolt to the release position, locking means which is rendered active or inactive by key operation from the inner side of the assembly and which when active renders the first actuator inoperable, and a second actuator at an outer side of the assembly which is operable to move the latch bolt to the release position.
In normal latch assemblies of the foregoing kind operation of the outer or second actuator does not affect the operation of the locking means. That locking means remains active in spite of operation of the outer actuator, and can be rendered inactive only by appropriate and deliberate operation of the locking means from the inner side of the latch assembly. It is usually the case in such prior assemblies that the locking means is key operated. That is, the locking means will generally include a key operated tumbler lock and locking mechanism connected to that lock so as to be influenced by operation of the lock. The second or outer actuator may also include a key operated tumbler lock, but in conventional assemblies of the foregoing kind operation of that lock does not influence operation of the first actuator locking means. The two locks are arranged so that one is operated from the inner side of the assembly and the other is operated from the outer side.